Method of retrieving anchors

ABSTRACT

A method of retrieving an anchor buried in the sea bottom and attached by a long mooring line to a floating moored structure. A submerged hook is towed with a service line in a direction intersecting the mooring line at a point between the floating structure and the anchor until the hook catches the mooring line. The service line is then swerved away from the structure in the direction of the mooring line with the hook sliding down on the mooring line until the hook arrives at or near the anchor. The service line is then pulled up to unseat the anchor. The service line is gradually reeled in with the progression of the anchor retrieval process.

United States Patent [1 1 Montgomery Dec. 30, 1975 [75] Inventor: Thomas M. Montgomery, Aberdeen,

[73] Assignee: Ocean Drilling & Exploration Company, New Orleans, La.

[22] Filed: Nov. 11, 1974 [21] Appl. No.: 522,395

[52] U.S. Cl 114/206 R [51] Int. Cl. 1363B 21/26 [58] Field of Search 114/206 R, 230, 221 R,

114/50, 235 R, 235 B, 235 F, 210, 43.5, .5 D; 61/723, 69 R; 294/66 R [56] References Cited UNITED STATES PATENTS 1,973,719 9/1934 Lake 61/69 R 2,338,067 12/1943 Wicklow 114/206 R 2,551,998 5/1951 De Groot.. 294/66 R 2,594,120 4/1952 Busher 294/66 R 3,097,495 7/1963 Laborde et a1. 114/.5 D

Slonczewski 6l/72.3 Lunde 114/206 R Primary Examiner-Trygve M. Blix Assistant Examiner-Stuart M. Goldstein Attorney, Agent, or Firm-Michael P. Breston [57] ABSTRACT A method of retrieving an anchor buried in the sea bottom and attached by a long mooring line to a floating moored structure. A submerged hook is towed with a service line in a direction intersecting the mooring line at a point between the floating structure and the anchor until the hook catches the mooring line. The service line is then swerved away from the structure in the direction of the mooring line with the hook sliding down on the mooring line until the hook arrives at or near the anchor. The service line is then pulled up to unseat the anchor. The service line is gradually reeled in with the progression of the anchor retrieval process.

5 Claims, 8 Drawing Figures US. Patent Dec. 30, 1975 Sheet 1 of 2 FAIRLEADER MOORlNG LINE PENDANT LINE MOORNG LINE PENDANT LINE 22 (PRIOR ART) FIGII US. Patent Dec. 30, 1975 Sheet 2 of2 3,929,087

METHOD OF RETRIEVING ANCHORS BACKGROUND OF THE INVENTION Marine structures are frequently moored to the sea bottom by anchors connected to the structure by a spread of mooring lines. Drilling vessels are illustrative of such marine structures for which the mooring system is both critical and very expensive. With the greater emphasis on offshore exploration, drilling rigs will now be required to operate within water depths of up to and over 3,000 feet. The mooring system is designed to maintain the rig within certain horizontal limits from the center line of the well. Dynamic anchors are presently employed because they increase their holding power with the pull provided by the rig.

To prevent an uplifting force from becoming exerted on and unseating the anchor, a sufficient length of mooring line must be deployed, considerably greater than the water depth. Maximum holding power is obtained when the fluke angle is set at approximately 30 for sandy bottoms and 50 for rocky bottoms.

Mooring lines consisting of wire rope, chain, or a I suitable combination of rope and chain have been successfully used in proportions depending on several factors which include: expected mooring line loads, water depth, handling equipment, storage facilities on board the drilling rig, and types of work boats available for assisting the rig during the deployment and retrieval process of the mooring system.

The location of each anchor in a spread mooring system is marked by a surface or marker buoy connected to the anchor by an anchor cable, known as a pendant line. A submerged assist or spring buoy can also be connected to the pendant line to provide a taut line from the anchor. In practice, a portion of the pendant line drags over the sea bottom, especially when the surface buoy becomes subjected to strong winds. Abrasion resulting from such dragging can cause a pendant line to prematurely fail.

Various shackles are used to join sections of pendant lines, to attach chain to wire rope, and to attach chain or wire rope to anchors. When long pendant lines are reeled in, the various connecting devices have a tendency to crush the wire rope. The work boat must chase" the mooring line attached to the anchor whose pendant line fails in order to first locate and then retrieve the lost anchor.

The chasing of a mooring line was accomplished in prior practice with the use of a bridle at the end of a service line by: manually bridling the mooring line near the water surface, guiding the bridle on the mooring line down to or near the anchor, unseating the anchor by uplifting the service line, gradually reeling in the service line as the rig brings in the mooring line, and close to the water surface manually disconnecting the bridle when the anchor is bolstered to the rig.

It will be appreciated that the rig, after it is moored, extends a considerable distance below the water surface. The mooring line is paid out of a chain locker, over a wildcat, and through a fairleader down to the sea bottom. Since the mooring line must be manually bridled and unbridled in the known chasing process at a point below but close to the lowermost fairleader, which can be fifty or more feet below the water surface, there is a need to first debalast the rig in order to raise the fairleader to near the water surface, and then manually position the bridle on and removing it from the mooring line. These hand manipulations are very time consuming, difficult and expensive. They also require bringing the work boat dangerously close to the drilling rig, a maneuver that can damage the drilling rig, the work boat, as well as harm their respective crews.

It is an object of the present invention to avoid the need to debalast the rig and to have to bring the work boat too close to the semi-submersible drilling rig in order to position the bridle on and removing it from the mooring line near the water surface. It is also an object of this invention to considerably reduce the time required to carry out the anchor chasing process.

SUMMARY OF THE INVENTION A method of retrieving an anchor and/or mooring line by towing a submerged hook at the end of a service line, in a direction intersecting the mooring line at a point remote from the moored structure. The book catches the mooring line at a point considerably below the water surface. The service line is gradually swerved in the direction of the mooring line toward the anchor, while maintaining tension on the service line. When the hook slides down to or near the buried anchor, the service line is raised and the hook unseats the buried anchor. When the anchor is already close to being bolstered on the rig, the hook is allowed to fall off from the mooring line by slackening the service line.

The hook preferably has a straight portion followed by an annular portion and then by a lip portion. The mouth or gap between the straight and lip portions has a dimension which is greater than the maximum dimension of any section of the mooring line to allow the hook to slide down on the mooring line.

Since the hook can automatically catch the mooring line at a point which is at a considerable distance below the water surface, there is neither a need to debalast the semi-submersible rig nor to maneuver the work boat within dangerous range of the rig.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1A shows a marine structure moored in accordance with the prior art by using a mooring line, an anchor, a pendant line, and buoys;

FIG. 1B shows a prior art chasing bridle;

FIG. 1C illustrates the prior art use of a work boat for retrieving an unmarked anchor;

FIG. 2 shows a preferred hook for practicing the method of this invention;

FIG. 3 is a sectional view on line 3-3 in FIG. 2;

FIG. 4 shows the trajectory of the work boat towing the hook of FIG. 2;

FIG. 5 shows the position of the hook of FIG. 2 immediately prior to catching the mooring line; and

FIG. 6 shows the movements of the work boat in the anchor retrieval process.

DETAILED DESCRIPTION OF THE INVENTION The prior art is illustrated in FIGS. lA-IC. Throughout the drawings the same numerals are used to designate the same or similar parts.

A floating structure 10 such as a submersible, shipshape or barge drilling rig 11 has a chain locker 12 which stores a very long mooring line 14 which may consist of wire, chain, or a combination of both. The mooring line is considerably longer than the depth of the body of water to allow for a nearly horizontal pull on the anchor. This pull causes the dynamic anchor 16 to bite deep into the sea bottom 18. Mooring line 14 is paid out over a wildcat 19 which is mounted at an elevated position relative to chain locker 12. The mooring line extends below a bottom fairlead 20 positioned at a considerable distance, say 50 feet, below wildcat 19. The crown 21 of anchor 16 is attached to a long pendant line 22 which is coupled to a marker surface buoy. When pendant line 22 is very long, it is also typically coupled to a spring buoy 26. The spring buoy supports a considerable portion of the weight of the pendant line 22.

It quite frequently happens that the pendant line 22 ruptures at a point 27. The rupture of the pendant line can be caused by several factors including abrasion, corrosion, excessive winds hurling the surface buoy 24, mishandling of the pendant line 22 on the winches, etc. When the pendant line and its spring and marker buoys become detached, anchor 16 is said to be lost.

To find and retrieve a lost anchor, and/or mooring line, prior practice involved raising to the surface the bottom fairlead 20 from a depth of say more than 50 feet to a depth of no more than a couple of feet below the water surface in the case of a semi-submersible. Rig 11 must therefore be debalasted so as to raise the fairleader.

At the start of the chasing process, a work boat 28 is then brought dangerously close to the marine structure 10, in order to manually latch bridle 30 unto mooring line 14 with a catch bolt 32 (FIG. 1C). Catch 32 is connected to a service line 34 paid out from a winch or Windlass 36 on the work boat 28.

After manually latching the mooring line with bridle 30 near the water surface, the work boat moves away in the direction of the mooring line toward anchor 16. When bridle 30 arrives at or near buried anchor 16, winch 36 reels in the service line 34 to thereby exert an uplift force on and unseat the anchor 16 from bottom 18.

Toward the end of the chasing, the work boat must again return dangerously close to the drilling rig. Typically, the work boat reassists rig 11 in the taking in of the mooring line into locker 12 and in the bolstering of the anchor. Bridle 30 is manually unlatched and removed from the mooring line by removing bolt 32 (FIG. 1C).

It will be appreciated that especially in rough seas, the manual coupling and uncoupling of hook 30 on and from mooring line 14 is both a very time consuming and hazardous operation. Should the service boat and rig collide, great damage is likely to result to both structures with the risk of serious injury to their crews.

After having briefly described the prior practice, the great advantages achievable with the present invention will now become apparent from the following description with reference to FIGS. 2-6.

In accordance with this invention there is now provided a hook, generally designated as 40. The hook can assume various geometric configurations. The preferred embodiment however, consists of a straight portion 41, an annular portion 42, and a lip portion 43. The gap or mouth 44 between portions 41 and 43 has a width sufficient to allow hook to grapple any section of mooring line 14. The hooks annular portion 42 is provided with a reinforcing rib 45 which conveniently can have a rectangular cross-sectional area (FIG. 3) while the hook itself can have a circular crosssection.

It is preferred that the weight distribution of hook 40 be such that it will ride on the mooring line with its 4 mouth 44 pointing in a downward direction toward bottom 18. The dimensions and weight of hook 40 are such that it will retain its structural integrity under maximum pulling load.

In operation, the work boats service line 34 is connected by a suitable shackle 34' to the hooks straight portion 41. Hook 40 is towed by work boat 28 in a direction 46 that intersects or is substantially perpendicular to the vertical plane containing mooring line 14. While being towed at the end of service line 34, hook 40 is submerged at a considerable distance below the water surface, say more than 200 feet, thereby obviating the need of the prior practice for debalasting in the case of a semi-submersible rig 11, and for having to bring work boat 28 too close to rig 11.

Hook 40 will catch mooring line 14 at a point 50 well below the lowermost fairlead 20, hence work boat 28 can maintain a safe distance from rig 11. After hook 40 grapples the mooring line at point 50, the work boat will continuously maintain tension on hook 40 while continuing to move in the general direction.46 but gradually swerving toward the direction of the mooring line, as indicated by the dotted line trajectory 48. Such gradual swerving will allow tension to be maintained on line 34 which will prevent hook 40 from falling off from mooring line 14.

As the work boat approaches buried anchor 16, it moves in a direction 49 substantially parallel to the mooring line, causing hook 40 to slide down on the mooring line until it stops at or near the anchor. Then, service line 34 is gradually reeled in by which 36 on work boat 28 to thereby exert an uplift force on the anchor 21. This force ultimately unseats the anchor from sea bottom 18.

Thereafter the work boat carries the anchor on its service line toward rig 11 to finish the retrieval process of the mooring line and of the anchor. To disconnect hook 40 from mooring line 14, all that is necessary in accordance with this invention is to slacken service line 34 and hook 40 will automatically fall off by its own weight from the mooring line.

It can now be appreciated that the coupling and uncoupling of hook 40 to and from mooring line 14 are accomplished when work boat 28 is at a safe distance from the moored structure 10, which could be any marine structure and this invention is, therefore, not limited to a drilling rig. Also, such coupling and uncoupling can now be carried out in a matter of minutes as compared to hours previously required with bridle 30 (FIG. 1C).

Other advantages and modifications will readily suggest themselves to those skilled in the art and all such are desired to be covered by the claims attached hereto.

What is claimed is:

l. A method for locating and retrieving an anchor from the seabed and being attached to the lower end of an inclined mooring line which is considerably longer than the depth of the water body, the upper end of the mooring line being attached to a member of a floating marine structure, comprising the steps of:

attaching to one portion of a long service line a hook having an open gap;

submerging the hook in the water;

towing a remote portion of the service line in a direction intersecting the inclined mooring line, the length between said portions of the service line being such as to allow the submerged hook to catch the mooring line at a point considerably remote from said structure;

gradually swerving the service line in the direction of the mooring line toward the buried anchor while maintaining sufficient tension on the service line and maintaining the hook in engagement with the mooring line as the hook slides down on the mooring line; and

exerting an uplift force on the service line sufficient to retrieve the anchor.

2. A method for locating and retrieving an unmarked anchor buried deep in the seabed and being attached to the lower end of a mooring line which is considerably longer than the depth of the water body, the upper end of the mooring line being attached to a member of a semi-submersible drilling rig, said member being at a considerable distance below the upper water surface,

whereby the entire mooring line is fully submerged,

suspended, and inclined in the body of water, comprising the steps of:

attaching to one portion of a long service line a hook having an open gap; submerging the hook in the water; towing a remote portion of the service line on the upper water surface in a direction intersecting the mooring line, the length of the submerged portion of the service line being such as to allow the submerged hook to catch the mooring line at a point considerably remote from said member;

gradually swerving the service line in the direction of the mooring line toward the buried anchor while maintaining sufficient tension on the service line and maintaining the hook in continuous engagement with the mooring line as the hook slides down on the mooring line with its gap facing toward the seabed;

when the hook arrives in the proximity of the seabed,

exerting an uplift force on the service line sufficient to lift the buried anchor into the water body; retrieving the anchor;

gradually bringing the mooring line into the drilling slaciening the service line; and

allowing the hook to automatically unhook from the mooring line.

3. The method of claim 2 wherein said hook is submerged at a distance greater than 200 feet below the upper water surface while being towed by said service line.

4. The'method of claim 2 wherein said hook has a straight portion, a curved portion, and a substantially straight lip portion, the gap between the straight and lip portions having a width dimension which is greater than the largest cross-sectional dimension of said mooring line.

5. The method of claim 4 wherein said curved portion has an outwardly extending rib. 

1. A method for locating and retrieving an anchor from the seabed and being attached to the lower end of an inclined mooring line which is considerably longer than the depth of the water body, the upper end of the mooring line being attached to a member of a floating marine structure, comprising the steps of: attaching to one portion of a long service line a hook having an open gap; submerging the hook in the water; towing a remote portion of the service line in a direction intersecting the inclined mooring line, the length between said portions of the service line being such as to allow the submerged hook to catch the mooring line at a point considerably remote from said structure; gradually swerving the service line in the direction of the mooring line toward the buried anchor while maintaining sufficient tension on the service line and maintaining the hook in engagement with the mooring line as the hook slides down on the mooring line; and exerting an uplift force on the service line sufficient to retrieve the anchor.
 2. A method for locating and retrieving an unmarked anchor buried deep in the seabed and being attached to the lower end of a mooring line which is considerably longer than the depth of the water body, the upper end of the mooring line being attached to a member of a semi-submersible drilling rig, said member being at a considerable distance below the upper water surface, whereby the entire mooring line is fully submerged, suspended, and inclined in the body of water, comprising the steps of: attaching to one portion of a long service line a hook having an open gap; submerging the hook in the water; towing a remote portion of the service line on the upper water surface in a direcTion intersecting the mooring line, the length of the submerged portion of the service line being such as to allow the submerged hook to catch the mooring line at a point considerably remote from said member; gradually swerving the service line in the direction of the mooring line toward the buried anchor while maintaining sufficient tension on the service line and maintaining the hook in continuous engagement with the mooring line as the hook slides down on the mooring line with its gap facing toward the seabed; when the hook arrives in the proximity of the seabed, exerting an uplift force on the service line sufficient to lift the buried anchor into the water body; retrieving the anchor; gradually bringing the mooring line into the drilling rig; slackening the service line; and allowing the hook to automatically unhook from the mooring line.
 3. The method of claim 2 wherein said hook is submerged at a distance greater than 200 feet below the upper water surface while being towed by said service line.
 4. The method of claim 2 wherein said hook has a straight portion, a curved portion, and a substantially straight lip portion, the gap between the straight and lip portions having a width dimension which is greater than the largest cross-sectional dimension of said mooring line.
 5. The method of claim 4 wherein said curved portion has an outwardly extending rib. 